Chlorination of polyester structure



March 3, 1959 L. E. WOLlNSKl 2,876,185

CHLORINATION OF POLYESTER STRUCTURE Filed Sept. 6. 1956 ULTRAVIOLET LAM PS) REACTION OHAMBER INVENTOR LEON E. WOLINSKI ATTORNEY factory heat-seal characteristics thereof.

CHLORINATION OF POLYESTER STRUCTURE Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application September 6, 1956, Serial No. 608,324

5 Claims. (Cl. 204-158) This invention relates to improvements in synthetic linear polyester structures and, more particularly, to an improved method for enhancing the heat-seal characteristics of oriented film of polyethylene terephthalate and .like synthetic linear polymers.

Films of polyethylene terephthalate and like linear polyesters which have been biaxially-o-riented, i. e., stretched and/0r rolled in two directions, find use in a great variety of applications, e. g., packaging, electrical applications, as a dielectric, protective coverings, glass replacements, etc. Heretofore, use of such films has been greatly restricted in applications requiring heatsealing or lamination of the film because of the unsatis- When it is attempted to heat-seal or bond together through the agency of heat, an oriented polyethylene terephthalate film, the film shrinks or retracts in the heated area and the heat-seal is usually puckered, and may also be brittle and/or opaque, and the film is liable to tear at the seam as it shrinks. Furthermore, it is necessary to employ,

even to effect an unsatisfactory seal between layers of oriented film, temperatures far above normal heat-sealing temperaturesemployed in sealing unoriented thermoplastic films. For example, whereas unstretched polyethylene terephthalate film is heat-scalable at temperatures from 150 C.-l70 C., it is necessary to employ heat-sealing temperatures within the range of from 235 C.-245 C. to seal oriented polyethylene terephthalate film which has been stretched three times (3X) in both directions and heat-set (i. e., subjected to heat at from 150 C.-200 C. while under tension).

It is an object of this invention toprovide a method for materially improving the adhesion, and particularly the heat-seal characteristics of snythetic linear polyester structures, e. g., films. A further object is to provide an improved biaxially-oriented polyethylene terephthalate film which may be satisfactorily heat-sealed at moderate temperatures (190 C. or less) to form attractive, strong and Water-insensitive seals. These and other objects will more clearly appear hereinafter.

By water-insensitive seals is meant that the seals retain approximately the original seal strength after 60 hours immersion in water at about 25 C.

The foregoing objects are realized by the present invention which, briefly stated, comprises subjecting the surface of synthetic linear polyester structures, e. g., oriented polyethylene terephthalate film, to the action of a gaseous atmosphere consisting essentially of chlorine.

as the sole reactive ingredient in the presence of ultraviolet light having wave-lengths of 2500 to 3200 A. under conditions of time and temperature such that the chlorine content of the surface of the treated film does not exceed the percent by Weight of chlorine, based on the Percent ehlorine=i where T is the thickness of the film in mils.

United States atet Leon E. Wolinski, Buifalo, N. Y., assignor to E. I. do

The preferred subject for treatment in accordance with the process of this invention is biaxially-oriented, balanced, heat-set, polyethylene terephthalate, i. e., polyethylene terephthalate film, which has been drawn to the same extent in both the longitudinal (machine) direction and transverse direction of the film and thereafter heatset by known expedients. It is to be understood, how ever, that the present invention comprehends oriented films of any synthetic linear polyester prepared by reacting terephthalic acid, dialkyl terephthalates, or esterforming derivatives thereof with a glycol ofthe series HO(CH ),,OH, where n is a whole number Within the range of 2-10. They may also include up to 20% by weight of a second acid or ester thereof, said second acid being selected from the group consisting of isophthali'c acid, dibenzoic acid, hexa-hydro-terephthalic acid, adipic acid, sebacic acid, azelaic acid, naphthalic acid, 2,5-dimethyl terephthalic acid, and bis-p-carboxyphenoxy ethane.

An essential feature of the present process is that the chlorination reaction be catalyzed by ultra-violet light having wave-lengths within the range of from 2500 to 3200 A. I have discovered that ultra-violet light within this range accelerates the reaction greatly. For example, the reaction time required to surface-chlorinate oriented polyethylene terephthalate film to the extent of 0.1% chlorine by the action of chlorine gas in the presence of ultra-violet light having wave-lengths within the range of 3200 to 3900 A is about one hour, whereas, if the same photo-chemical reaction is carried out in the presence of ultra-violet light having wave-lengths within the range of 2500 to 3200 A., the reaction time is reduced to 1 second or less. The use of ultra-violet light of wave-lengths below the herein-specified range may result in serious degradation of the film.

Another factor of critical importance is the maximum degree of surface chlorination. Theheat-seals of polyethylene terephthalate film containing a greater percentage of chlorine than is defined by Formula 1 above-are water-sensitive, i. e., the seals fail completely after im mersion in water at room temperature for a period of 60 hours. The time required to effect surface chlorination of the film to the desired degree will vary in known manher with the temperature of treatment. The time required to effect a substantial improvement in the heatseal characteristics and to produce a film which will yield Water-insensitive heat-seals, is as low as 0.1 of a second when the film is subjected to an atmosphere containing chlorine maintained at a temperature of about 255 C., or as low as 2 seconds at room temperature (25 ,C.). On the other hand, the treatment under the same conditions of chlorine concentration and temperature may be continued for as long as 60 seconds at 255 C., or 300 seconds at 25 0, without excessive chlorination of the film as above-specified.

A preferred method of chlorination is illustrated diagrammatically in the accompanying drawing. Referring to the drawing, continuous film from any desirable source, e. g., film fed directly from the film stretching apparatus, or from a mill roll is passed between a bank of ultra-violet lamps mounted in a closed reaction chamber. Chlorine gas in introduced into the chamber by suitable gas inlet means at one end of the chamber and is exhausted at the opposite end. If desired, the film may be preheated in a heating zone just prior to the entry of the film into the reaction chamber. The chlorinated film is continuously collected on a conventional film windup roll. The linear speed of the film through the reaction chamber will, of course, be adjusted in accordance with the pre-determined temperature of treatment and the degree of chlorination desired.

contact time of 2 seconds.

3 It is preferred that the concentration of chlorine gas in the reaction chamber be substantially 100%. Although the content of chlorine gas may fall far below,

e. g., below this value, and the benefits of the in- -a cool drum maintained at 65 C. After quenching, the

filmwas stretched 3X in both directions in accordance with the method set forth in copending application of A. C. Scarlett, United States Serial No. 287,354, filed May 12, 1952, and heat-set while held under tension at 150 C. The film had a final thickness of 0.5 mil. After heat-setting, the film was ready to be subjected to the treatment which will be described in the following examples of specific embodiments which will serve to illustrate the principles and practice of the present invention.

EXAMPLE 1 Samples of 0.5 mil polyethylene terephthalate film,

:prepared as described above, were placed on a chome ferrotype plate. This plate was then placed under the lamps in a reaction chamber containing two General Electric A-H, mercuryarc lamps with the glass shields .removed, a thermometer, and an inlet and an outlet for -the reaction gas. lamps with the outer glass units removed were found to have the following active wave-lengths in Angstrom 'unitsr'2535, 2649, 2801, 2872, 2964, 3019, 3124, 3340, 3645, and above.

The General Electric A-H, mercury Lamps with the glass shields intact were found to cutoff all wave-lengths below 2964 Angstrom units.

"Prior to insertion of the film sample, chlorine gas was conducted into the chamber through the inlet and the chamber was flushed for 10 minutes with the gas so as to remove any impurities. The concentration of the chlorine in the reaction chamber was essentially 100%.

The non-preheated film samples (one surface) were then subjected to the action of the chlorine gas for varying "reaction times. The film was then tested for heat-sealability, and water-sensitivity. Heat-seal strength is a measure of the strength of the bond between two films when they are sealed together by heat and pressure.

For purposes of comparison and definition, the follow- 'ing test is used to measure the strength of the heat-seal bond.

A piece of film 4" x 10", with the grain (machine direction) running in the long direction, is cut into two pieces, 4" x 5", handling all pieces by the corner so as not to contact the areas to be sealed. The two pieces 4" x 5" are superimposed on the others so that the opposite (chlorinated) surfaces are in contact. The two pieces of superimposed film are then sealed together at-each end, at right angles to the grain with a sealing bar. The sealing bar was 0.75" wide, and the film was heated to 190 C. with lbs/sq. in. pressure, and a The sealed sheets are then cut in half at right angles to the grain, and each half is cut into 1.5" wide strips, "parallel to the grain, from the center of the sheet, resulting in four sets to be tested. Each of the 1.5" wide seal strips is opened at the free ends, placed in a Suter testing machine and pulled apart. The force in grams required to pull the strips apart is taken as a measure of the heat-seal bond strength.

The sensitivity of the treated heat-sealed film samples 'to water was-tested by immersing the film sample in water kept at room temperature. After 60 hours, the

4 t film was removed from the water and the seal tested in a Suter tester.

Table 1, below, lists the effect of varying reaction times on the heat-scalability and water-sensitivity of these film samples. Listed are the approximate reaction time in seconds, the initial film temperature, the reaction chamber temperature, the average heat-seal in grams, the sensitivity of the seals after 60 hours, and the percent chlorine in the film determined by analysis of the film.

Table I CHLORINA'IION 0F POLYETHYLENE TEREPHTHALATE FILM Initial Reaction Film Charn- Heat- Water-Sensi- Raaction Time Temperber Seal, tivity After Percent (seconds) ature, TempergJinch 60 Hours Chlorine C. aturc,

28 53 480 Insensitive 28 55 28 53 23 54 28 56 28 59 28 65 300 60 1 (Control)...

1 Lamps shielded with window glass which filters out ultra-violet light below 3200 A.

EXAMPLE 2 Samples of 0.5 mil polyethylene terephthalate film, prepared as described above were preheated to 150 C. and thereafter treated in a similar manner to that described in Example 1. Table II lists the approximate reaction time in seconds, the initial film temperature, the reaction chamber temperature, the average heatseal in grams, the sensitivity of the seals to water after 60 hours, and the percent chlorine determined by analysis of the film.

Table II CHLORINATION OF POLYETHYLENE TEREPHTHALATE FILM Initial Reaction Film Oharn- Heat- Water-Sensi- Reaction Time Temperher Seal, tivity After Percent (seconds) ature, TempergJinch 60 Hours Chlorine C. ature,

150 64 453 Insensitive. 150 64 443 do 0 23:i;.03 150 65 .4 =l-;.04 150 68 $.03 150 72 5:. 10 150 77 $.08

Although in the embodiments of the invention described hereinbefore, the film has been stretched in two directions and heat-set prior to undergoing halogenation, it is to be understood that the film may be subjected to the action of halogen gas at any time after being extruded.

While the method of the present invention is particularly suitable for use in connection with synthetic polyester films, such as polyethylene terephthalate film, it can be applied with advantage to other structures, as well, such as fibers, filaments, tubes, rods, and the like; the surfaces of which are desired to he modified so that they can be satisfactorily subjected to scaling operations. Although particular emphasis, heretofore, has been placed on the preparation of films for heatsealing operations, it is to be understood that theprocess of the invention is no less successful in producing films which are readily. sealed by other means such as solvent adhesives, cements and glues. Films produced by the present process are also highly receptive-because of their improved adhesion to coating operations wherein coatings designed to improve such properties as the moistureproofness, resistance to corona discharge, etc., are applied to the film.

The process, as set forth hereinbefore, presents a quick, efiicient route for imparting highly satisfactory adhesive properties to synthetic linear polyester film structures. The present method radically speeds up the rate of reaction of the chlorine on the surface of the linear polyester structure, over methods now known to the art. The catalysis of the reaction characteristic of the process of the present invention with ultra-violet light rays having Angstrom units within the range of 2500- 3200 A. presents a novel, yet remarkably practical, method of obtaining good adhesion, e. g., heat-sealability, dye-receptivity, in linear polyester structures without sacrificing the inherent desired physical. characteristics of these structures.

The advantages of readily dyeable polymers of ethylene terephthalate and related polymers to manufacturers of apparel and other textiles is readily apparent, as is the utility of adhesiveness and printability of films so comprised to makers and users of packaging and similar materials. Polyethylene terephthalate fabrics treated with ozone or manufactured from staple fibers so treated, have an added desirable characteristic of increased resistance to pilling, i. e., tendency toward the accumulation of numerous unsightly small balls of yarn on the surface of the fabric. Other benefits of the practice of the present invention will come readily to mind.

The availability of sealable, oriented, polyethylene terephthalate films opens many new uses for such films heretofore not useful because of the non-adherability of these types of films. Such markets as the pre-packaging of meats, frozen food packaging, bags, etc., will now be open to the use of the subject films and the like, prepared by the process of the present invention.

I claim:

1. The process of treating film of synthetic linear polyesters of a glycol of the series HO(CH ),,OH wherein n is a whole number within the range of 2-10 inclusive and acids selected from the group consisting of (l) terephthalic acid and (2) a mixture of terephthalic acid with up to 20% by weight, based on the total Weight of acids, of a second acid selected from the group consisting of isophthalic acid, dibenzoic acid, hexahydroterephthalic acid, adipic acid, sebacic acid, azelaic acid, naphthalic acid, 2,5-dimethyl terephthalic acid and bisp-carboxyphenoxy ethane, which comprises subjecting the surface of said film to the action of a gaseous atmosphere consisting essentially of chlorine as the sole reactive ingredient, in the presence of ultra-violet light having wave-lengths of 2500 to 3200 A. under conditions of time and temperature such that the chlorine content of the surface of the treated film does not exceed the percent by weight of chlorine, based on the weight of the treated film, defined by the equation:

Percent ehlorine='%,

where T is the thickness of the film in mils.

2. The process of claim 1 wherein the gaseous atmosphere consists of chlorine.

3. The process of claim 2 wherein the polyester is polyethylene terephthalate.

4. The process of claim 3 wherein the film is oriented polyethylene terephthalate film.

5. The process of claim 4 wherein the film is biaxiallyoriented polyethylene terephthalate film.

References Cited in the file of this patent UNITED STATES PATENTS 2,715,075 Wolinski Aug. 9, 1955 

1. THE PROCESS OF TREATING FILM OF SYNTHETIC LINER POLYESTERS OF A GLYCOL OF THE SERIES HO(CH2)NOH WHERIN N IS A WHOLE NUMBER WHERIN THE RANGE OF 2-10 INCLUSIVE AND ACIDS SELECTED FROM THE GROUPE CONSISTING OF (1) TEREPHTHALIC ACID AND (2) A MIXTURE OF TEREPHTHALIC ACID WITH UP TO 20% BY WEIGHT BASED ON THE TOTAL WEIGHT OF ACIDS, OF A SECOND ACID SELECTED FROM THE GROUP CONSISTING OF ISOPHTHALIC ACID, DIBENZOIC ACID, HYDAHYDROTEREPHTHALIC ACID SEBACIC ACID, AZELAIC ACID, NAPHTHALIC ACIS, 2.5-DIMETHYL TEREPHTHALIC ACID AND BISP-CARBOXYHENOXY ETHANE, WHICH COMPRISE SUBJECTING THE SURFACE OF SAID FILM TO THE ACTION OF A GASEOUS ATMOSPHERE CONSISTING ESSENTIALLY OF CHLORINE AS THE SOLE REACTIVE INGREDIENT, IN THE PRESENCE OF ULTRA-VIOLET LIGHT HAVING WAVE-LENGTH OF 2500 TO 3200 A. UNDER CONDICTIONS OF TIME AND TEMPERATURE SUCH THAT THE CHLORINE CONTENT OF THE SURFACE OF THE TREATED FILM DOES NOT EXCEED THE PERCENT BY WEIGHT OF CHLORINE, BASED ON THE WEIGHT OF THE TREATED FILM, DEFINED THE EQUATION. 